This lesson will explain how substances move across a cell membrane through the process of diffusion as well as discuss why this process is important in our bodies.

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Source: Video and Images Created by Amanda Soderlind

Video Transcription

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In this lesson today, we are going to discuss the process of diffusion. So simply put, diffusion is the movement of molecules down a concentration gradient. So I'm going to take a moment to explain what a concentration gradient is. Concentration gradient is basically the difference in the number of molecules of a substance across a space. So when a substance is diffusing, it's moving from an area of high concentration to an area of low concentration.

So diffusion is the movement of molecules from an area of high concentration to an area of low concentration without using any of the cell's energy. So diffusion is a type of passive transport, just meaning that it doesn't require any energy from the cell in order for this to happen. So the purpose of diffusion basically is just to even out the concentration of molecules across the space.

So let's take a look at an example of diffusion using food coloring. So we have a fishbowl right here. And we're going to say that I drop some green food coloring into the fishbowl. Now when you first drop the food coloring into the fishbowl, it'll all be kind of clumped up in that space right where you dropped it.

But slowly, those molecules are going to start to spread out. They're going to move to an area of high concentration, right where they were dropped in, to an area of low concentration. The rest of this for this fishbowl doesn't have any green food coloring molecules. So this would be an area of low concentration out here, and this would be an area of high concentration.

So those molecules are going to spread out from that area of high concentration to that area of low concentration. And when diffusion is done, those molecules will be equally spread or equally distributed throughout the water in that fishbowl. And so you would notice, when you first dropped the food coloring in, you would see just this area of green. But after diffusion occurs, all of the water would then be green, and you would know that those molecules had spread out evenly, or diffused evenly, from an area of high to low concentration.

So here's another example in a cell. Outside of the cell, we might have a certain solute that we have in high concentration. We have a lot of it outside of the cell. We don't have very much of it inside of the cell. So naturally, those molecules are going to diffuse from an area, again, of high concentration to low concentration.

You're going to see these molecules out here diffuse across the plasma membrane into the cell. So again, when diffusion has happened, you would have an equal number of molecules outside as you would inside. So these will continue to diffuse into the cell until it's equal on either side of the cell.

So this lesson has been a brief overview on the process of diffusion and how molecules will move from an area of high concentration to an area of low concentration naturally, without the use of cell energy.

Terms to Know
Concentration Gradient

The difference in concentration of a substance between two areas


Movement of molecules from an area of high concentration to an area of low concentration

Passive Transport

The movement of molecules across a membrane without the use of energy